The present invention relates generally to hydraulic lash compensators (or adjusters), and more particularly to a hydraulic lash adjuster which is of extremely light weight and cost effective construction.
Hydraulic lash adjusters for internal combustion engines have been in use for many years to eliminate clearance, or lash, between engine valve train components under varying operating conditions in order to provide a consistent valve motion and to maintain engine operating efficiency and to reduce noise and wear in the valve train. Hydraulic lash adjusters operate on the principle of transmitting the energy of the valve actuating cam through hydraulic fluid trapped in a pressure chamber behind a plunger. During each revolution of the cam, as the length of the valve actuating components varies due to temperature changes or wear, small quantities of hydraulic fluid are permitted to enter or escape from the pressure chamber and thus effect an adjustment in the position of the plunger, and consequently, an adjustment of the effective total length of the valve train.
The cam operating cycle comprises two distinct events: base circle and valve actuation. The base circle event is characterized by a constant radius between the cam center of rotation and the cam follower during which effectively no motion or cam energy is transmitted. The valve actuation event is characterized by a varying radius between the cam center of rotation and the cam follower which effectively transmits cam energy to open and close an engine valve. During the valve actuation event, a portion of the loads due to the valve spring, the inertia of valve train components, and cylinder pressure are transmitted through the valve train and through the lash adjuster. These loads raise the pressure of the hydraulic fluid within the lash adjuster pressure chamber in proportion to the plunger area, and in current hydraulic lash adjusters, causes some fluid to escape from the pressure chamber. As the fluid escapes, the plunger moves down according to the change in volume of the pressure chamber, shortening the effective length of the valve train. During the base circle event, the lash adjuster plunger spring moves the plunger up such that no clearance or lash exists between valve actuation components.
Hydraulic fluid is drawn into the pressure chamber through the plunger check valve in response to the increased volume of the pressure chamber as the plunger moves up. If the effective length of the valve train shortens during the cam operating cycle, positive lash is created and the lash adjuster extends, moving the plunger to a higher position at the end of the cycle than at the beginning. Inversely, if the effective length of the valve train lengthens during the cam cycle, negative lash is created and the lash adjuster contracts, moving the plunger to a lower position at the end of the cycle than at the beginning. The latter condition typically occurs when valve train components lengthen in response to increased temperature.
In prior art hydraulic lash adjusters the escape of hydraulic fluid from the pressure chamber is between the plunger and the wall of the lash adjuster body. Such escape or "leakdown" is controlled solely by the fit of the plunger within the body. Effective operation of the lash adjuster requires that the leakdown be precisely controlled, and thus, the fit between the plunger and the body must be held to a very close clearance, e.g., between about 0.000200 in. (0.00508 mm) and 0.000230 in. (0.00584 mm). Such close clearances require selective fitting of the plunger to the body, which is an expensive operation.
U.S. Pat. No. 5,622,147, assigned to the assignee of the present invention, and incorporated herein by reference, discloses a hydraulic lash adjuster wherein the fit between the plunger and the body is relatively loose in comparison with prior art designs, but wherein a resilient seal between the plunger and the body is used to maintain a pressure seal between the high and low pressure regions of the lash adjuster. Effective leakdown is obtained by providing a normally open check valve and closely controlling the movement of the check valve between its open and closed positions, wherein during the initial portion of the valve actuation event some hydraulic fluid escapes from the high pressure chamber as the flowing fluid closes the check valve. The plunger then moves downward according to the change in volume of the pressure chamber, thus shortening the effective overall length of the valve train.
The elimination of the need for leakdown between the plunger and the lash adjuster body, and the precision fit required thereby, provides an opportunity to make fundamental changes in the plunger and body structures permitting the use of significantly lighter weight and more economically fabricated components than heretofore considered possible. The present invention addresses this opportunity by providing a lash adjuster body and a plunger which can be fabricated by a cold forming process which produces the parts to essentially net shape requiring little or no additional finishing steps prior to assembly. The thin-walled structure produced by cold forming further permits certain ports and passages, as well as other characteristics required for the function of the lash adjuster, to be formed as part of the cold forming process; whereas, in prior art designs additional machining steps were required to obtain these characteristics.